1. Field of the Invention
The present invention relates to optical disk drives, particularly, to sled motor control for optical disk drives.
2. Description of the Related Art
In general, optical disk drives, such as Compact Disk (CD) drives and Digital Video Disk (DVD) drives, include a sled servo system (or a coarse actuator system) for achieving coarse tracking of the optical pickup. The sled servo system moves the optical pickup across the optical disk to make the optical pickup follow the spiral track on the optical disk. A typical sled servo system is composed of sled motor and a rotation-reciprocation conversion mechanism.
One of the typical methods for controlling the sled servo system is to generate a sled drive signal, used for driving the sled motor, from a tracking drive signal, used for driving the tracking actuator within the optical pickup. The tracking drive signal is typically generated through phase compensation of the tracking error signal. The low frequency components of the tracking drive signal is representative of the general displacement of the tracking actuator, and the sled drive signal is generated through phase compensation of the low frequency components of the tracking drive signal. The sled drive signal thus generated effectively achieves positioning the optical pickup so that the displacement of the tracking actuator is coarsely close to 0.
One requirement on the sled servo control is to reduce the duration required for “kick operation”, that is, track jump operation for seeking the track. Japanese Laid-Open Patent Application No. P2001-167451A discloses that the sled motor control system is switched to open-loop control and the sled motor is then driven with an acceleration pulse having an increased pulse height and width. Japanese Laid-Open Patent Application No. P2001-167448A discloses that a kick pulse and a break pulse is superposed to the sled drive signal. Japanese Laid-Open Patent Application No. H10-255286A disclosed that the dead zone for driving the sled motor is eliminated during the kick operation to thereby rapidly move the optical pickup.
Another requirement is to stably control the sled motor. One cause of the instability in the sled motor control is that the optical pickup may be stuck due to the increased friction within the sled servo mechanism. Japanese Laid-Open Patent Application No. H10-255285A discloses that the sled servo system detects undesirable sticking of the optical pickup from the differential of the sled drive signal, and increases the sled drive signal so that the that the optical pickup gets rid of the sticking.
Another cause is that the mechanical load of the sled motor may be fluctuated. For example, the mechanism for converting the rotation of the sled motor into the reciprocation of the optical pickup may suffer from the change over time in the static friction therein.
Japanese Laid-Open Patent Application No. H05-189778A discloses an optical pickup feed device for achieving stable control of the sled servo system against the fluctuation of the mechanical load of the sled motor. This optical pickup feed device generates drive pulses in response to the low frequency components of the tracking actuator, and drives the sled motor by using the drive pulses, not by using continuous drive signals. The use of the drive pulses for driving the sled motor provides stable control over the fluctuation of the load, and is effective for improving the stability of the sled servo system. Additionally, Japanese Laid-Open Patent Application No. H05-189778A discloses that the pulse height and width of the drive pulses are modified when the low frequency components of the displacement of the tracking actuator is not changed, or the changed amount thereof is small.
Driving the sled motor with drive pulses requires appropriate adjustment of the waveform of the drive pulses, specifically, the pulse height and width of the drive pulses. The unsuitable waveform of the drive pulses may cause an undesired behavior of the optical pickup. An unsuitable waveform of the drive pulses may cause sticking of the optical pickup, or may cause undesirably large travel of the optical pickup.
One of factors for hindering the appropriate control of the pulse height and width of the drive pulses is the variation in the characteristics of the sled motor. Optical pickups experience different travel distances for the same pulse height and width due to the inevitable variation of the sled motor. At the extreme, a certain sled motor may be rotated and the other sled motor may not be rotated, for the same pulse height and width of the drive pulses. Particularly, the problem of the characteristics variation in the sled motor is serious when an inexpensive DC motor is used as the sled motor,